Waveguide displays are particularly useful in providing a means for displaying an image in a region of limited space. In a waveguide display, image-bearing light is injected into a waveguide and is subsequently released from the waveguide for viewing, for example a helmet-mounted or head-up display device.
In such devices, image-bearing light may be focussed at infinity, and a viewer views/receives the released light directly. In this way, the viewer focusses on an image seen by looking at/through the transparent waveguide output area and this permits light from an ambient, external and real-world scene to also enter the viewers eye through the waveguide output area (without being guided). In this way, a desired image may be seen to overlay the external scene. Optical waveguides for display devices of this type often initially insert image-bearing light into the waveguide by a process of diffraction, such that inserted light propagates along the waveguide by total internal reflection towards an output part arranged to release guided light from the waveguide by a further process of diffraction, for viewing. These waveguides may be slim, such as a slab waveguide or rod waveguide, and therefore occupy relatively little volume. FIG. 1 schematically shows an example of this. A slab optical waveguide 1 is orientated vertically and receives image-bearing light 2 at an uppermost input region 3 of the waveguide, generally above the viewer 4, for guiding to a lower output region 5 generally in front of the viewer, in a direction towards a viewer.
Light 6 from an external scene is able to pass through the output part of the waveguide towards the viewer such that the image-bearing light overlays the external scene as seen by the viewer.
Because the output direction of image-bearing light output from the output part of the waveguide is generally perpendicular to the plane of the waveguide 1, this dictates that the waveguide must be generally vertical to permit output light to be viewed in the horizontal—a desired orientation in head-up displays. This can be problematic where there is limited vertical space/height available in a vehicle/aircraft, or on a helmet, for accommodating the waveguide.
An alternative arrangement is shown in FIG. 2 in which the waveguide 1 is orientated in the horizontal plane above the viewer, who no longer views the output part of the waveguide directly. A combiner 7 is located in front of the viewer and comprises a flat sheet of material (e.g. a glass) generally reflective to the image-bearing light while being generally transmissive to external light 7 from an external scene. The combiner is thereby able to combine image-bearing light (reflected) and light from an external scene (transmitted) for concurrent viewing by the viewer.
However, this overhead positioning of the waveguide can lead to head-clearance limitations and problems for a viewer. Furthermore, the distance between the waveguide output part and the combiner results in an increased path length for image-bearing light from the output of the waveguide to the viewer's eye. This has the result of reducing the instantaneous field of view seen by the viewer.
The invention aims to provide an improved waveguide display.